Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#91 Pipeline Pigging

5 min read

Pipeline Pigging refers to the practice of using devices or implements known as ‘pigs’ to perform various cleaning, clearing, maintenance, inspection, dimensioning, process and pipeline testing operations on new and existing pipelines. For existing operational pipelines pigging is normally performed without stopping the flow of the product in the pipeline.

Pipeline pigging is a concept in pipeline maintenance that involves the use of devices known as pigs, which clean pipelines and are capable of checking pipeline condition. This process is done without necessarily interfering with the flow of product in the pipe. Pipeline pigs are used in a number of different industries including oil & gas, lube oil, chemical plants and hygienic applications such as pharmaceutical or food.  Selecting the right pipeline pig for a particular application requires extensive experience.

The primary purpose of pipeline pigs is to make sure that the pipe is clean and free from obstruction. The pig is usually cylindrical or spherical to aid movement and efficient cleaning.  As the pig moves through the pipeline, it can remove and possibly detect any build-ups within the pipe which can often lead to reduced performance, increased energy costs and cause corrosion, which could lead to disastrous leaks and cracks in the pipe.

The pipeline pig is inserted into the pipe using a pig trap, which is typically oversized to accommodate the tight fitting pig. A similar pig trap or receiver is located at the end of the pipe to receive the pig once it has traveled the length of the pipeline.

A pig will often make a sound as it travels through a pipe and if it doesn’t, a Transmitter can be installed into the pig. The pig can then be tracked through the pipe from above ground using an Receiver.

Pigs may be used in hydrostatic testing and pipeline drying, internal cleaning, internal coating, liquid management, batching, and inspection.

Hydrostatic testing

Pigs are used during  hydrostatic testing operations to allow the pipeline to be filled with water, or other test medium, without entrapping air. The pig is inserted ahead of the fill point, and water is pumped behind the pig to keep the pipe full of water and force air out ahead of the pig. Pigs are then used to remove the test waters and to dry the pipeline.

Pipeline cleanup

Operations may conduct pigging on a regular basis to clean solids, scale, wax buildup (paraffin), and other debris from the pipe wall to keep the pipeline flow efficiency high. In addition to general cleaning, natural-gas pipelines use pigs to manage liquid accumulation and keep the pipe free of liquids. Water and natural-gas liquids can condense out of the gas stream as it cools and contacts the pipe wall and pocket in low places, which affects flow efficiency and can lead to enhanced corrosion.

Batch transportation

Pigs are used in product pipelines to physically separate, or “batch,” the variety of hydrocarbons that are transported through the line. Product pipelines may simultaneously transport gasoline, diesel fuel, fuel oils, and other products, which are kept separated by batching pigs.

Prevention of solid accumulation and corrosion

Crude-oil pipelines are sometimes pigged to keep water and solids from accumulating in low spots and creating corrosion cells. This can be especially necessary when flow velocities are less than 3 ft/sec. Multiphase pipelines may have to be pigged frequently to limit liquid holdup and minimize the slug volumes of liquid which can be generated by the system.

Coating

Pigs may be used to apply internal pipe coatings, such as epoxy coating materials, in operating pipelines. Pigs may also be used with corrosion inhibitors to distribute and coat the entire internal wetted perimeter.

Inspection

Pigs are being used more frequently as inspection tools. Gauging or sizing pigs are typically run following the completion of new construction or line repair to determine if there are any internal obstructions, bends, or buckles in the pipe. Pigs can also be equipped with cameras to allow viewing of the pipe internals. Electronic intelligent, or smart, “pigs” that use magnetic and ultrasonic systems have been developed and refined that locate and measure internal and external corrosion pitting, dents, buckles, and any other anomalies in the pipe wall.

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